U.S. patent number 4,356,376 [Application Number 06/263,235] was granted by the patent office on 1982-10-26 for pulse laser pretreated machining.
This patent grant is currently assigned to General Electric Company. Invention is credited to Robert J. Douglas, Donald G. Flom, Marshall G. Jones, Ranga Komanduri, Minyoung Lee, Robert A. Thompson.
United States Patent |
4,356,376 |
Komanduri , et al. |
October 26, 1982 |
Pulse laser pretreated machining
Abstract
An improved laser-assisted machining process has special
application to difficult-to-machine materials such as the titanium
alloys and high temperature superalloys. A layer of material to be
removed by a cutting tool is made weaker by drilling a series of
holes using a pulse laser beam ahead of the cutting process so that
the tool removes the rest of the weaker material with relative
ease. There is a decrease of cutting forces, breakage of the chip
to a manageable size, and reduced tool wear.
Inventors: |
Komanduri; Ranga (Schenectady,
NY), Lee; Minyoung (Schenectady, NY), Flom; Donald G.
(Scotia, NY), Thompson; Robert A. (Quaker Street, NY),
Jones; Marshall G. (Scotia, NY), Douglas; Robert J.
(Schenectady, NY) |
Assignee: |
General Electric Company
(Schenectady, NY)
|
Family
ID: |
23000930 |
Appl.
No.: |
06/263,235 |
Filed: |
May 13, 1981 |
Current U.S.
Class: |
219/121.72;
219/121.67; 219/121.68; 219/121.69; 219/121.7; 219/121.71;
219/121.84; 29/27C; 82/1.11 |
Current CPC
Class: |
B23K
26/0823 (20130101); B23P 25/006 (20130101); Y10T
82/10 (20150115); Y10T 29/5114 (20150115) |
Current International
Class: |
B23K
26/08 (20060101); B23P 25/00 (20060101); B23K
027/00 () |
Field of
Search: |
;219/121LN,121LG,121LH,121LK,121LL,121FS,121L,121LM ;82/1C,DIG.1
;29/27C,558 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Michael Bass, et al., American Institute of Physics, "Laser
Assisted Hot Spot Machining", pp. 205-211, 1979. .
Mayfield, Aviation Week and Space Technology, "Advanced Machining
Processes, Tooling", pp. 54-56, Sep. 15, 1980..
|
Primary Examiner: Albritton; C. L.
Attorney, Agent or Firm: Campbell; Donald R. Davis, Jr.;
James C. Snyder; Marvin
Claims
The invention claimed is:
1. A method of machining a workpiece made of a difficult-to-machine
material comprising:
pretreating the outer surface of the workpiece prior to the cutting
process by using a pulse laser beam to drill circumferentially
spaced holes in a layer of material to be removed, said holes
having a depth no greater than a known depth of cut of a cutting
tool; and
rotating said workpiece relative to said cutting tool to remove the
layer of material which is weakened by said holes and to generate a
segmented chip.
2. The method of claim 1 wherein the diameter of said pulse laser
drilled holes is greater than the width of said chip.
3. The method of claim 1 wherein the depth-to-diameter aspect ratio
of said pulse laser drilled holes is 1 to 2.
4. The method of claim 1 wherein said pulse laser beam is generated
by a 1.06 micrometer wavelength laser.
5. A method of machining a workpiece made of a difficult-to-machine
material such as the titanium alloys and high temperature
superalloys, comprising:
rotating the workpiece and pretreating its outer surface prior to
the cutting process by using a pulse laser beam to drill a series
of radial holes in a layer of material to be removed, said holes
having a depth no greater than a known depth of cut of a
single-point cutting tool and a diameter greater than an expected
chip width; and
removing, with said cutting tool, the pretreated layer of material
which has been weakened by said series of holes, and generating a
segmented chip.
6. The method of claim 5 wherein said pulse laser beam is generated
by a neodymium-YAG laser operated in pulse mode.
7. The method of claim 5 wherein said pulse laser beam is generated
by a neodymium-YAG laser having a repetition rate of at least 400
pulses per second.
8. The method of claim 5 wherein said pulse laser drilled holes
have a depth between 0.020 inch and 0.060 inch and a diameter
between 0.010 inch and 0.060 inch.
9. The method of claim 5 wherein the workpiece is bathed with a gas
to enhance material removal by said pulse laser beam.
Description
BACKGROUND OF THE INVENTION
This invention relates to an improved method of machining the
difficult-to-machine materials.
Certain materials such as the titanium alloys and high temperature
superalloys are extremely difficult to machine due to their
inherent chemical and physical properties; among the former are the
high chemical reactivity with the tool material and the
environment, and among the latter are high yield strength at
temperatures close to the melting temperature of the alloys
together with poor thermal properties. In fact, the verty
properties that render the superalloys suitable for high
temperature applications are responsible for their poor
machinability. Consequently, in most cases, these materials have to
be machined at lower speeds (45-125 surface feet per minute). For
this and other machining applications there are additional
restrictions on the machine tools such as the vibrations that
affect finish, limited availability of power that afffects the
removal rate, etc. All this reduces the productivity and increases
the cost.
One way of improving the efficiency of machining these materials is
to weaken them by a secondary operation, such as by heating. In the
past, researchers have suggested various methods of heating
including induction heating, furnace heating, resistance heating,
and plasma arc heating, but lack of control of the small area to be
heated was the main drawback with these sources. A continuous wave
laser, with its small spot size and supposedly better focusing
ability, offers an alternative source for heating a small area to a
small depth of cut, and can be used either to heat up the layer to
be machined off just prior to the arrival of the material into the
shear zone or at some convenient small lag angle (up to
10.degree.). The former weakens the material by heating it locally
to high temperature but may have some drawbacks including higher
tool-chip interface temperature and consequent rapid tool wear,
lack of adequate focusing control of heat at or near the shear
zone, and generation of a long continuous chip which is difficult
to handle and can damage the machined part.
SUMMARY OF THE INVENTION
A laser operated in pulse mode is used to decrease the cutting
force by mechanically altering the outer surface of the
difficult-to-machine workpiece prior to machining with a cutting
tool. Pulse laser pretreatment ahead of cutting involves laser
vaporization and formation of a series of holes in the layer of
material to be removed, thus weakening the material for subsequent
machining. The laser is a neodymium-YAG (yttrium-aluminum-garnet)
pulse laser, or its equivalent, which has a high pulse rate such as
400 pulses per second. There is a significant reduction in cutting
forces (hence energy) when machining the material that is laser
predrilled. The reduction in energy is much more than due to
reduction in the volume of material due to laser predrilling.
Moreover, the chip that is removed is segmented and broken into
small pieces and there is reduced tool wear.
The depth of the radial holes in the layer of material to be
removed is less than the depth of cut of the cutting tool, and the
hole diameter is greater than the chip width and the cross feed of
the lathe. Preferably, the laser is mounted on the machine tool
carriage and the laser treats the workpiece while cutting; the
pulse laser beam impinges on the workpiece 90.degree. or less in
advance of the horizontal cutting tool. An alternative two-step
method is to drill rows of holes in the workpiece, followed by
machining on a lathe. Enhanced material removal by the pulsed laser
is realized by application of a gas such as oxygen.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic cross section of a workpiece with laser
predrilled holes which is machined by a cutting tool;
FIG. 2 is a partial perspective view of the pretreated workpiece
and cutting tool;
FIG. 3 illustrates prior art single point turning to remove a
continuous chip;
FIG. 4 illustrates pulse laser pretreated machining and the reduced
length of the shear plane;
FIG. 5 is a simplified perspective of a conventional lathe modified
to have laser equipment supported on and movable with the carriage;
and
FIG. 6 is a top view of a workpiece which has rows of laser drilled
holes in the layer that, as a second step, is machined off.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The titanium alloys, high temperature nickel-based superalloys, and
other difficult-to-machine materials have to be machined at low
speeds. Even at these lower speeds, if the feed and/or depth of cut
can be increased with the same cutting force, the amount of
material removal could be increased for the same amount of power
input. This is accomplished by reducing the shearing force on the
shear plane, and one way of doing this is by mechanical weakening.
A laser operated in a pulse mode is used to decrease the cutting
force by altering the surface prior to cutting with a single point
tool.
The workpiece 10 in FIGS. 1 and 2 is, for instance, a one inch
diameter 6-4 titanium (6 percent aluminum, 4 percent vanadium,
titanium alloy) rod. A series of circumferentially spaced holes 11
are formed by a focused pulse laser beam 12 in the outer surface of
the workpiece in a layer 13 of material to be removed by a cutting
tool 14. The depth of predrilled radial holes 11 is no greater than
the depth of cut of the cutting tool, and the depth to diameter
aspect ratio of the holes is between 1 and 2. The hole diameter is
greater than the cross feed per revolution of the cutting tool and
is larger than the expected chip width. The pulse laser 15 is a
neodymium-YAG laser, or its equivalent, having a minimum repetition
rate of 400 pulses per second. The beam is focused by an objective
lens 16 to a small spot at the surface of the workpiece. Since the
laser treatment of the workpiece is vaporization of surface
material, the objective lens must be protected. The cutting tool
such as tungsten-carbide tool insert 14 is mounted on a tool holder
17 which has a conventional built-in or external chip breaker 18,
and meets the workpiece 90.degree. from laser beam impingement. The
chip is automatically broken by the cutting action, and chip
segments 19 may be disposed of easily.
The length of the shear plane is reduced by this pulse laser
pretreatment ahead of the cutting process, and hence there is a
reduction in cutting force. There is, in fact, as much as a 50
percent reduction of cutting force. Conventional single-point
turning with a lathe will produce a continuous chip as is
illustrated in FIG. 3. A solid layer of material is removed from
the surface of workpiece 10' and the chip 20 is long and unbroken.
The shear pulse indicated at 21 extends diagonally and has a length
l. The shear plane l' in the pulse laser predrilled case, FIG. 4,
extends vertically from the trailing edge of one predrilled hole 11
to the leading edge of the next hole, and has a shorter length. A
comparison of machining the solid layer with the laser predrilled
layer shows a significant reduction in forces, and hence energy,
when machining material that is laser pretreated. The reduction in
energy is much more than due to the reduction of the volume of
material due to laser predrilling. Tests confirming this were
conducted on a lathe at 100 surface feet per minute. Tool wear for
a tungsten-carbide tool material was also tested by making a
comparison between a tool used for pulse laser assisted machining
and a tool used without lase assist. A crater was worn into the
face of the carbide tool during non-laser cutting, but when laser
assisted machining was used, no crater was formed with identical
cutting conditions. There is a tool wear improvement as well as a
significant reduction in forces.
The size of the predrilled holes depends on the material being
machined. The diameter of the holes is typically 0.010 inch to
0.060 inch and their depth is 0.020 inch to 0.060 inch. It is
preferred that the depth of cut is slightly deeper than the hole
depth. The cross feed per revolution of the cutting tool is
typically 0.006 inch and the chip width is usually somewhat greater
than the cross feed. The hole diameter, as was mentioned, is
greater than the expected chip width. In the circumferential
direction, the predrilled holes in experiments that have been
conducted are about 1/32 inch apart at work speeds of roughly 60
sfpm, which is typical for the superalloys such as Inconel 718.
Instead of directing the pulse laser beam vertically downward to
the workpiece, the laser beam impingement can be less than
90.degree. in advance of the cutting tool. If the holes are laser
drilled close to the point of contact of the cutting tool, the
material is also weakened by the heating effect and there is a
gain.
FIG. 5 shows a conventional lathe which is modified to perform
pulse laser assisted machining. The pulse laser beam and cutting
tool are synchronized and move together, and this is accomplished
by mounting the laser head and associated optics rigidly to the
carriage of the lathe. A Raytheon high pulse rate 400 watt Nd-YAG
laser is interfaced with a 9 inch Monarch lathe. This 1.06
micrometer wavelength laser is capable of repetition rates of 400
pulses per second with a maximum average power output of 125 watts;
the maximum energy generated is approximately 0.4 joules per pulse
at 400 pps. Energy per pulse can be increased with decreasing pulse
rates. The headstock of the lathe is generally indicated at 24, the
tailstock at 25, the workpiece at 26, the horizontally movable
carriage at 27, and the tool holder and cutting tool at 28. A laser
equipment table 29 is supported above the lathe, on the
horizontally movable carriage 27, by means of struts 30-32. Laser
head 33 generates the laser beam 34 which is deflected by a mirror
35 and focused onto the surface of the workpiece by the objective
lens 36. A plastic film or tape (not shown) moves continuously
between the objective lens and workpiece to protect the former
during laser assisted machining. There is flexibility for aligning
the laser beam and cutting tool in the same plane which is normal
to the workpiece axis of rotation. There is also a vertical
adjustment of the objective lens which enables variation in the
laser focused spot size and therefore the resulting power density.
A binocular microscope is used to locate the focal plane.
Experiments that were performed verified the decrease of cutting
forces, breakage into short chips, and reduced tool wear during
pulse laser assisted machining. In performing these cutting tests,
the laser was operating at an average power output of approximately
125 watts, and the laser beam pulse width (pulse length) was 200
microseconds while the pulse rate was 400 pps. The average energy
pulse under these conditions is approximately 0.31 joules/pulse.
The resulting peak power per pulse is 1.6 kilowatts and the
corresponding power density is 2.times.10.sup.5 watts/cm.sup.2.
Typical energy density per pulse is approximately 40
joules/cm.sup.2. These parameters result from a cutting speed of 63
sfpm.
Oxygen was used during certain test runs to increase laser material
vaporization. The workpiece was bathed in a low flow of oxygen
cover gas delivered by tube 40, FIG. 1, and this increased the
percent force reduction. The use of this and other gases may be
beneficial during this treatment.
An alternative but less desirable two-step method involves
initially treating the workpiece with a pulse laser followed by
machining on a lathe. Referring to FIG. 6, circumferentially
spaced, axially extending rows of holes 38 are drilled in the
surface of workpiece 39 as it is rotated, in the layer of material
to be machined, by a pulse laser. The holes in every row usualy
overlap one another. Subsequently, the laser pretreated workpiece
is machined on a lathe. This is the full equivalent of having the
pulse laser treat the workpiece while cutting, and the same
advantages of reduced cutting forces and energy, chip breakage, and
improved tool wear are realized.
Either method can be implemented with two or more pulse lasers that
are synchronized. High pulse rates are readily achieved in this
manner. Continuous wave lasers, it will be noted, cannot be
cascaded.
Other information and photographs are given in Aviation Week and
Space Technology, Vol. 113, No. 11, Sept. 15, 1980, pp. 54-56.
While the invention has been particularly shown and described with
reference to preferred embodiments thereof, it will be understood
by those skilled in the art that various changes in form and
details may be made therein without departing from the spirit and
scope of the invention.
* * * * *